Aqueous solution for and process for producing surface layers on iron and steel articles



AQUEOUS SGLUTION FOR AND PROCESS FOR PRODUCING SURFACE LAYERS N IRON AND STEEL ARTICLES Friedrich Rossteutscher, Frankfurt am Main, Germany, assignor to Farbwerke Hoechst Aktiengeseilschaft vorrnals Meister- Lucius & Bruning, Frankfurt am Main, Germany, a corporation of Germany N0 Drawing. Application March 19, 1954 Serial No. 417,515

Claims priority, application Germany December 12, 1949 9 Claims. (Cl. Ids-6.15)

The present invention relates to a process for producing surface layers on iron and steel articles.

The objective .of my present invention is to produce a strongly acid aqueous solution suitable for producing surface layers on iron and steel articles. This solution contains one or several alkali metal chlorates, oxalic acid radicals and phosphoric acid radicals, and as cations potassium, sodium and ammonium. As alkali metal chlorates there come into consideration, for instance, potassium chlorate or sodium chlorate.

Surface layers on iron and steel articles can advantageously be produced by exposing the surfaces of these articles to the action of such a strongly acid aqueous solution. It is to be understood that the term phosphoric acid radicals comprises the radicals of all the acids which may be obtained by hydration of phosphorus pentoxide such as, for instance, meta-phosphoric, orthophosphoric, pyro-phosphoric, tripoly-phosphoric, tetrapoly-phosphoric acid and other poly-phosphoric acids. In addition to the ingredients named above the treating solution contains potassium, sodium or ammonium in the form of cations.

The oxalic acid and/ or phosphoric acid radicals may be present in the solution in the form of oxalic acid or of one of the phosphoric acids mentioned above. When such acids are present in the solution, it contains as cations potassium or sodium, e. g. in a quantity corresponding to that of the alkali metal chlorate. Moreover, it is also possible further to add to the solution alkali metal or ammonium radicals in the form of their soluble salts as, for instance, their chlorides and sulfates. The oxalic acid and/or phosphoric acid radicals may also, either totally or partially, be present in the form of potassium, sodium or ammonium salts of the above named oxalic and/or phosphoric acid. Oxalic acid radicals in the form of oxalic acid may, ofcourse, likewise be present in addition to phosphoric acid radicals in the form of alkalic metal or ammonium salts of the above named phosphoric acids or vice versa.

The strongly acid solutions have a pH-value ranging from about 1.0 to about 2.5. It is of particular advantage to use solutions having a pH-value from between 1.5 and 2.0. By an appropriate selection of the acid ingredients and the quantity of said ingredients contained in the treating solution, the latter may be adjusted to the desired pH-value.

Thus, for instance, a treating solution is used which contains, at the beginning of the treatment, per 1 mol of oxalic acid radical about 1 atom of the cation, e. g. sodium, potassium or ammonium. The oxalic acid radical is used up by the reactions occurring during the treatment; thereby the proportion of alkali radical to oxalic radical is altered in favor of the alkali radical. It is of importance that the cations sodium, potassium 2,832,707 Patented Apr. 29, 1958 or ammonium are present in a sufiicient quantity so that the insoluble FePO, formed during the production of the surface layer is transformed into a soluble form with formation of a complex salt, thus avoiding with certainty the undesirable formation of sludge in the treating bath, a fact which is of particular advantage. This advantage resides especially in the fact that it allows of operating continuously Without it being necessary, as it is the case with the working. methods hitherto known, to discontinue the working operation from time to time in order to remove the sludge from the bath container.

No sludge formation occurs in the solution even when it is permanently used.

When repeatedly used, the solution shows a certain reduction of its ingredients since they have been used up for the production of the surface layer. For this reason the used up substances are replaced at a rate corresponding to their consumption by introducing solutions containing the corresponding ingredients into the original solution. I

The treatment according to the invention may be carried through as follows: the article to be treated, either made of iron, steel or alloy steel, as for instance sheet iron, is dipped into the solution. During this dipping process the bath liquid may be agitated by means of mechanical devices or by blowing in air or other gas. The treatment may also be carried out in a stationary bath by moving the articles to be coated. As articles to be treated there come into consideration, in addition to sheet iron, for instance, small working pieces such as screws, bolts, etc. They are most advantageously treated by placing them into a perforated drum which slowly rotates in the bath (rotation rate about /2 to 2 per minute). Bulky and large pieces are treated by placing them into Wire baskets and dipping the latter into the treating solution or by fixing the articles to wire and dipping them into the bath liquid. The treatment may also be effected by spraying the solution by means of nozzles on the surface of the articles, it being of advantage that in the absence of sludge formation clogging of the nozzles is avoided. It is understood that automatic sprayers may be applied.

The following is a further possibility of treating the article: it is introducedinto the solution and, while treating it, the liquid is removed by pumping at one or several places and then reintroduced by pumping into the bath container at one or several other places.

When applying the solution according to the invention a surface layer containing iron oxalate, in addition to iron phosphate, is produced on iron and steel articles. The layers so obtained have a fine crystalline structure and a grey to black coloration; they mayalso' be produced in the form of very thin layers having a thickness of about 1 1 to about 2,1. This is of special advantage for layers which are needed for small articles as they are used in the construction of typewriters or telephone apparatus. Moreover, the surface layers according to the invention are of adavntage if the treated articles are exposed to bending stress or when, in additionto improving the corrosion resistance, special importanceis attached to facilitating the application of color or lacquer coatings. By using solutions containing an ainmonium salt of oxalic acid and/or an ammonium salt of one of the above named phosphoric acids, layers having an especially marked dark coloration are obtained.

Surface layers according to the invention can be produced at room temperature, i. e. at temperatures ranging from 15 C. to 20 C. The process can also be effected at elevated temperatures up to about 50 C. At temperatures essentially exceeding 50 (3., operation is disturbed by the formation of sludge. The proportion of the concentrations of the compounds contained in the solutions may vary within wide limits. Thus, for instance, the proportion of oxalic acid radical to phosphoric acid radical may vary within the limits of about 1:3 and about 221, that of oxalic acid radical to chlorate radical within thelimits of about 1:1 and about 3:1, and that of phosphoric acid radical to chlorate radical within the limits of about 4:1 and about 1:2.

The process according to the invention may be applied on iron and any kind of non-alloyed steel; however, alloy steels having minor proportions of other elements, e. g. proportions of up to about 3 percent, may also come into consideration. There may be named by way of example nickel-steel'having a content of nickel of up to 3 percent.

The time of reaction during which the articles to be treated are subjected to the action of thesolution depends on the composition of the iron or steel, the nature of the surface, and the purpose which the treated workingpiece shall serve. As a rule the articles need only be treated for a short time until the reaction is complete. The treatment is finished as soon as a noticeable layer having the desired thickness has been formed. Quite generally, it is suflicient, when operating at a temperature of between about 15 C. and 20 C., to treat the surfaces for several minutes, i. e. l-5 minutes. An increase ofthe temperature does not require a substantial alteration of the treating period. Especially when treating alloy steels having minor proportions of up to about 3 percent of other elements, it may be necessary to pro long the time of treatment, for instance, to about to about minutes. Before starting the treatment, itis in most cases advisable to pretreat the metal surfaces by thoroughly cleaning and degreasing them. The degreasing may be accomplished with known agents, for instance, with agents containing meta-silicate, carbon tetrachloride, trichlor-ethylene, etc. The articles may be after-treated by thoroughly rinsing them with water in order to remove adhering ingredients of the bath; thereupon the articles may be dried by means of hot air.

It is of particular advantage to produce surface layers by means of strongly acid solutions having a pH-value between about 1.0 and 2.5 and containing (1) a member of the group consisting of oxalic acid, and alkali metal and ammonium oxalate in a concentration of from about 5 to about 25 grams per liter, (2) an alkali metal chlorate in a concentration of from about 7 to about 15 grams per liter, (3) a member of the group consisting of ortho-phosphoric, pyro-phosphoric, meta-phosphoric, -tripoly-phosphoric,-tetrapoly-phosphoric acid, alkali metal and primary ammonium ortho-phosphate, pyro-phosphate, meta-phosphate,"tripoly-phosphate, tetrapoly-phosphate in a concentration of from about 5 to about 60 grams per liter.

However, it is also possible to use solutions which also contain both oxalic acid and alkali metal or arm monium oxalate, these two substances together being present within the limits of about 5 to about 25 grams per liter of solution. Moreover, solutions which also contain both ortho-phosphoric or pyro-phosphoric or metaphosphoric or tripoly-phosphoric or tetrapoly-phosphoric acid and alkali metal or primary ammonium ortho-phosphates or pyro-phosphates or meta-phosphates or tripolyphosphates or tetrapoly-phosphates in a total concentration of about 5 to about 60 grams per liter, may likewise be used.

An especially advantageous solution contains, per liter of solution, for instance, 7-15 grams of sodium or potassium chlorate, 5-60 grams of sodium or potassium tripoly-phosphate and 5-25 grams of oxalic acid. The substances used for the preparation of this solution are solid products, thus simplifying the manipulation of the ingredients. However, it is also possible to use solutions in which the sodium or potassium tripoly-phosphate contained therein is replaced, either totally or partially, up to about 50 percent, by ortho-phosphoric acid.

A further possibility consists in using solutions which, at first, have a low content of alkali metal chlorate. During the treatment of the articles, alkali metal chlorate as a solid product or in the form of an aqueous suspension is introduced into the solution, continuously or discontinuously, at a rate corresponding to its consumption. The alkali metal chlorate contained in the starting solution may thus be decreased to a content of about 1 to about 2 grams per liter of solution.

Especially when hard water is used for thepreparation of the solution it is advisable, in order to prevent the precipitation of calcium oxalate, to add a stable inorganic acid which is stronger than the phosphoric acid and oxalic acid present in the solution. For instance sulfuric acid, calculated as H 50 of percent strength, may be used in a quantity of about 10 to about'30 grams per liter.

If alkalitripoly-phosphate or alkali tetrapoly-phosphate is present in the treating solution, an addition of sulfuric acid is unnecessary since, as a rule, these poly-phosphates prevent the precipitation of calcium.

For preparingt-he solutions according to this invention, the single components may be dissolved in water and added separately in the desired quantities. When the components have been added, it is advisable to stir the solution for some time, i. e. for about 5 to 10 minutes, inorder to obtain as homogeneous a solution as possible.

The components required for the preparation of the bath may be chosen so that all the substances are present in the-solid form. Thus, for instance, it is possible to pack these granular or powdery substances separately, one package containing alkali metal chlorate, for instance sodium chlorate, and alkali tripoly-phosphate, for instance sodium tripoly-phosphate, and the other package containing oxalic acid. Other solid substances suitable for the process of the present invention may also .be packed. In this case, the packages contain the substances in such quantities that, when they are dissolved in water, they are present in the proportion required for-the solution of this invention, i. e., a solution is obtained at the desired pH-value. The solid substances may be despatched in iron or wooden drums.

The process of producing surface layers on metal according to the present invention constitutes a great improvement over the hitherto known processes because-it is possible now to prepare the bath solution from solid substances.

The following examples serve to illustrate the invention, but .they are not intended to limit it thereto:

Example 1 '1 liter of a solution according to the invention is prepared for the treating bath. The solution obtained c on tains per liter 20.0 grams of oxalic acid -1 2.5 grams of sodium chlorate 5.0 grams of ammonium oxalate 10.0grams of primary ammonium phosphate and has a pH-value of 1.6 to 1.7.

A sheet iron of non-alloyed steel having a surface of cm? is cleaned in the usual manner and degreased with an agent containing sodium meta-silicate and then brought "into 1 liter of this solution. Duration of treatment: '3 minutes. Temperature of the bath: 20 C. When the treatment is complete, i. e. as soon as the surface 'layer'has been formed, the sheet iron is removed from the bath, thoroughly rinsed with water in order to remove the adhering ingredients of the bath, and dried with hot air. As a result of the treatment a dark grey layer has been formed on the surface. The thickness of this Iayer is 2a. The layerhaving a fine crystalline structure consists of equally formed crystals.

Example 2 1 liter of a solution according to the invention is prepared for the treating bath. The solution obtained contains per liter 10.0 grams of oxalic acid. 7

5.0 grams of ammonium oxalate 12.5 grams of sodium chlorate 50.0 grams of primary ammonium phosphate 10.0 grams of ortho-phosphoric acid calculated as H3PO4 of 100 percent strength, and has a pH-value of 2.1.

Y A sheet iron having a surface of 100 cm. is cleaned in the usual manner and degreased with an agent containing' sodium meta-silicate. 1 liter of the aforementioned solution is used for spraying (by means of a nozzle) said sheet iron. Duration of treatment: 2 minutes. Temperature of the bath: 20 C. When the treatment is complete, i. e. as soon as the surface layer has been formed, the sheet iron is thoroughly rinsed with water in order to remove the adhering ingredients of the bath, and dried with hot air. A black layer is obtained having a thickness of l-Z and a fine crystalline structure consisting of equally formed crystals.

Example 3 1 liter of a solution according to the invention is prepared for the treating bath. The solution obtained contains per liter 15.0 grams of oxalic acid 10.0 grams of sodium chlorate 10.0 grams of sodium tripoly-phosphate and has a pH-value of 1.6.

A sheet iron of non-alloyed steel having a surface of 100 cm. is cleaned and degreased with trichlor-ethylene and then treated in 1 liter of this solution. The sheet iron is brought into the solution and part of the solution is removed by pumping, i. e. the solution is filtered off with suction at one place and reintroduced into the bath at another place, so that the solution is constantly agitated. Duration of treatment: 2 minutes. Temperature of the bath: 22 C. When the treatment is complete, i. e. as soon as the surface layer has been formed, the sheet iron is removed from the bath, thoroughly rinsed with water in order to remove the adhering ingredients of the bath, and dried with hot air. A dark grey layer is obtained having a thickness of 1 /22 and a fine crystalline structure of equally formed crystals.

Example 4 1 liter of a solution according to the invention is prepared for the treating bath. The solution obtained contains per liter 22.7 grams of oxalic acid 7.2 grams of potassium chlorate 10.0 grams of sodium tripoly-phosphate 8.4 grams ofortho-phosphoric acid calculated as H PO of pH-value of 1.0.

A sheet iron of alloyed steel having a surface of 100 cm. and a content of nickel of 0.3 percent is cleaned in the usual manner and degreased with carbon tetrachloride and then treated with this solution. The treatment is effected by immersing the sheet iron for 5 minutes in the bath solution having a temperature of 24 C. When the treatment is complete, i. e. as soon as the surface layer has been formed, the sheet iron is removed from the bath, thoroughly rinsed with water in order to remove the adhering ingredients of the bath, and dried with hot air. A grey layer is obtained having a thickness of 1 and a fine crystalline structure consisting of equally formed crystals.

100 percent strength and has a Example 5 1 liter of a solution according to the invention is prepared for the treating bath. The solution obtained contains per liter 10.0 grams of oxalic acid 12.5 grams of sodium chlorate 7.5 grams ofortho-phosphoric acid calculated as H PO of 100 percent strength, and has a pH-value of 1.0.

A sheet iron having a surface of 100 cm. is cleaned in the usual manner and degreased with trichlor-ethylene and then immersed in '1 liter of this solution. Duration of treatment: 2. minutes. Temperature of the bath: 18 C. When the treatment is completed. e. as soon as the surface layer has been formed,- the'sheet iron is removed from thebath, thoroughly rinsed with water in order to remove the adhering ingredients of the bath, and dried with hot air. A grey layer is obtained having a thickness of 1.5; and a fine crystalline structure of equally formed crystals.

Example 6 1 liter of a solution according to the invention is prepared for the treating bath. The solution obtained contains per liter 10.0 grams of oxalic acid 7.2 grams of sodium chlorate 20.0 grams of primary ammonium phosphate and has a pH-value of 2.0.

A sheet iron having a surface of 100 cm. is cleaned in the usual manner and degreased with an agent containing sodium meta-silicate and then immersed in 1 liter of this solution. Duration of treatment: 5 minutes. Temperature of the bath: 20 C. When the treatment is complete, i. e. as soon as the surface layer has been formed, the sheet iron is removed from the bath, thoroughly rinsed with water in order to remove the adhering ingredients of the bath, and dried with hot air. A dark grey layer is obtained having a thickness of 1-2p. and a fine crystalline structure consisting of equally formed crystals.

This application is a continuation-in-part of application Serial No. 200,105, filed December 9, 1950, and now abandoned.

I claim:

1. A process for producing surface layers on iron and steel articles in a strongly acid bath which comprises exposing the surface of such article at a temperature ranging between about 15 C. and about 50 C. to the reaction of an aqueous solution having a pH of from about 1.0 to about 2.5 and containing per liter of solution (a) an alkalimetal chlorate in a concentration of from about 7 to about 15 grams, (b) a substance selected from the group consisting of oxalic acid, alkali metal oxalates, and ammonium oxalate, in a concentration of from about 5 to about 25 grams; and (c) a compound selected from the group consisting of meta-phosphoric, ortho-phosphoric, pyro-phoshoric, and poly-phosphoric acids, and the alkali metal and primary ammonium salts thereof in a concentration of from about 5 to about 60 grams.

2. The process of claim 1 wherein the pH of said aqueous solution is from about 1.5 to 2.0.

3. The process of claim 1 wherein the aqueous solution contains per liter from about 10 to about 30 grams of sulfuric acid calculated as by weight sulfuric acid.

4. The process of claim 1 wherein the surface of such article is exposed to the aqueous solution for from about 1 to 15 minutes.

5. A process for producing surface layers on iron and steel articles in a strongly acid bath which comprises exposing the surface of such article at a temperature ranging between about 15 C. and about 50 C. to the reaction of an aqueous solution having a pH of from about 1.0 to about 2.5 and containing per liter of solution from about 7 to 15 grams of an alkali metal chlorate, 5 to 60 grams of alkali metal tripoly-phosphate, and to 25 grams of oxalic acid.

.6- An -.a.qu ous OIutiQIt :for p ducing surfac lay ontim and s eel artic eslh qg pHof om o t 1.0 to about 2.5 and containing per liter of solution (a) an alkali metal chlorate in a concentration of from about 7 to about grams; (b) a substance selected from the group consisting of oxalic acid, alkali metal oxalates, and ammonium oxalate, in -a concentration of from about 5 to about grams; and (c) acompound selected 'from the group consisting of meta-phosphoric, ortho-phosphoric, pyrophosphoric, and poly-phosphoric acids, and the alkali metal and primary ammonium salts thereof in a concentrationof from about 51;) about grams.

7. Theaqueous solutionof claim ,6 wherein the pH'is from about 1.5 to 2.0.

8. The aqueous solution of claim 6 ,'where in the aqueous solu i n ontainspe lite from abou 1.0-1.0 ab ut 3.0

m8 grams of sulfuric acid calculated as by weight sulfuric acid. 1

-9- ,A aq eo s 0 t QnfQ p od c ng urfa lay onjronand s te el articles having a pH of fromabout 1.0 to about 2.5 and containing per liter o'f solution fromabout 7 to 15 grams of an alkali metal chlorate, 5 to 60 grams of alkali metal tripoly-phosphate, and 5 to 25 grains of oxalic acid. 7

References Gited in thefile of this patent UNITED STATES PATENTS 1,315,017 Gravell Sept. .2, 1919 1,895,569 .Curtin Jan. 31, 1933 2,164,042 Romig V. V June 27, 1939 2,236,549 Darsey et al. Apr. 1, 1941 2,318,656 T mp o --V-.,----.---.-- May .1 1 941 

1. A PROCESS FOR PRODUCING SURFACE LAYERS ON IRON AND STEEL ARTICLES IN A STRONGLY ACID BATH WHICH COMPRISES EXPOSING THE SURFACE OF SUCH ARTICLE AT A TEMPERATURE RANGING BETWEEN ABOUT 15*C. AND ABOUT 50*C. TO THE REACTION OF AN AQUEOUS SOLUTION HAVING A PH OF FROM ABOUT 1.0 TO ABOUT 2.5 AND CONTAINING PER LITER OF SOLUTION (A) AN ALKALIMETAL CHLORATE IN A CONCENTRATION OF FROM ABOUT 7 TO ABOUT 15 GRAMS, (B) A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF OXALIC ACID, ALKALI METAL OXALATES, AND AMMONIUM OXALATE, IN A CONCENTRATION OF FROM ABOUT 5 TO ABOUT 25 GRAMS; AND (C) A COMPOUND SELECTED FROM THE GROUP CONSISTING OF META-PHOSPHORIC, ORTHO-PHOPHORIC. PYRO-PHOSPHORIC, AND POLY-PHOSPHORIC ACIDS, AND THE ALKALI METAL AND PRIMARY AMMONIUM SALTS THEREOF IN A CONCENTRATION OF FROM ABOUT 5 TO ABOUT 60 GRAMS. 